The present invention generally relates to voice and data communication systems, and more specifically relates to a system where both voice and data signals are communicated on the same communications medium between a central office terminal and various subscriber terminals.
The most common method in use today of transmission of voice and data is a four wire communication system using frequency division multiplexing. However, it is often desirable to add a data signal to an existing voice or other analog information communications channel in order to transmit both signals independently and simultaneously over the same communications medium.
Attempts in the past to simultaneously transmit voice or video and data information simultaneously are represented by a number of issued patents. For example, U.S. Pat. No. 3,529,088 teaches a method of multiplexing sequential transmission of voice information and data information. Two patents, U.S. Pat. Nos. 3,894,190 and 4,238,849 teach a method of transmitting data over voice information by modulating digital message signals onto a subcarrier which lies in the ear psychological masking range of a simultaneously transmitted broadcast message. U.S. Pat. No. 3,406,344 teaches a method of modulating the voice carrier to transmit low frequency signals. U.S. Pat. No. 3,152,160 uses delay lines to transmit and detect transmitted signals which are decoded after the delay times to reconstruct the original signals. U.S. Pat. No. 3,838,444 teaches a method of altering a carrier signal which doubles the frequency of the carrier signal to indicate data transmission.
New techniques have been proposed to simultaneously transmit voice and data using a time compression multiplexing technique. However, such a time shared two wire digital subscriber transmission system, as proposed, requires the conversion of analog voice information into digital transmission and its use would require the conversion of telephone sets currently in use from analog to digital devices.